Electromagnetic radiation, and in particular various wavelengths of light, may be used to identify or sort various materials. One problem in material sorting relates to identifying gems. It can be difficult to identify different gems from the ore in which they are originally found or from various types of look alike material, such as glass or man made materials that may be used to imitate a gem, for example. Not all look-alike materials can be identified by any one test, and it is common in gemology to put a suspect material through a series of identification tests to properly identify and certify that the gem in question is genuine.
Many different techniques and devices exist for the identification of gems by fluorescence. The most prominent testing techniques involve using UV light or X-rays, although other excitation methods are also known. For example, U.S. Pat. No. 883,653 teaches using a blue incident light of a higher refrangibility to produce fluorescence of a lower refrangibility and to obviate the masking effect of the reflected light by using a second ray filter of the yellow colour to transmit red, orange yellow, yellow green and green fluorescence. However, in blue light, red orange yellow, green and green fluorescence are not necessarily masked by the blue light, and due to the large wavelength difference between the incident and fluorescing wavelengths, a simple or crude filter will work well to separate the incident light from the fluorescence.
U.S. Pat. No. 4,394,580 uses absorption, internal reflection and internal excitation to try to characterize gems. However, this invention teaches a wide source i.e. white light, and the use of red and green optical filters before a detector. However, the red and green gems could also be visible in the white spectrum light to the naked eye. Further, of course white light covers the visible light spectrum including red fluorescent wavelengths so any fluorescing will be masked by white light, even if a filter is used.
U.S. Pat. No. 5,118,181 relates to a method of identifying individual gemstones by using a series of specific incident light wavelengths in the ultraviolet or infra red spectrum and measuring the response of the gem stone to each separate wavelength. While useful to characterize a particular stone, this cannot be used in a mine environment to separate gem stones from ore for example.
Another previously known technique for identifying gems has been to detect the presence of fluorescing material such as found in certain gems containing the element Chromium. Such gems can red fluoresce when exposed to particular light frequencies, and examples of gems which can fluoresce red include emeralds, rubies and the like. In the past the fluorescence was detected by using ultraviolet light, or even an incident blue light on the gem in question. What has been previously known is that such incident light can elicit the red light fluorescence from specific gems, which could then be characterized based on the presence of such red light fluorescence. The blue incident light can be obtained by shining a white light through a blue liquid onto the object in question, although more recently it is more common to use a blue light source such as an LED. Longwave UV LEDs exist also. The technique uses incident light having wavelengths much shorter than the red light fluorescence, making the red light fluorescence easier to detect. It has also been understood up until now that while UV and blue light elicit a relatively strong and easily detectable red light fluorescence, wavelengths closer to the fluorescing wavelengths tend to exhibit a weaker and more muted excitation response. The use of a blue filter would of course eliminate red light wavelengths from the incident light. When the object is viewed through a red light filter, which blocks out the blue or UV incident light, the red fluorescence is then visible. In many cases strong fluorescence can be detected which is easily identified by eye, or even through the use of a crude filter which broadly transmits red light.
While blue or ultraviolet incident light works well in some circumstances, the use of these incident lights to elicit a red light luminescence is problematic in certain situations. For example, in underground mining of precious gem materials, it is not desirable for the mine workers to be able to easily identify the individual gemstones. These tend to be both small and very valuable so the use of any detecting method that makes the gems more apparent to the visible eye, or easily identifiable through the use of a crude filter is not desirable as valuable gemstones can go missing. So, blue, ultraviolet or even wide spectrum white lights all of which can make gem material visible to the naked eye, are not the most desirable form of incident light for sorting the gems from the background ore in a mine environment.
What is desired is a simple and easy way to use the red fluorescence of certain materials to characterize the material, without necessarily revealing the property of even the body colour of the material to the naked eye or making it easily detectable with a crude filter.